The well-known aromatic polyesters or copolyesters, in particular polyethylene terephthalate and the copolymers thereof with low proportions of, for example, isophtalic acid or cyclohexanedimethanol, polybutylene terephtalate, polytrimethylene terephtalate, polyethylene naphthalate and the copolyesters thereof, serving as starting materials for fibres, films, foils and packaging, are processed into granules of medium viscosity after a melt polycondensation. The mean degree of polycondensation, expressed as the intrinsic viscosity (I.V.) is in the range between 0.30-0.90 dl/g in case of polyethylene terephthalate and its correspondingly low-modified copolyesters after the melt polycondensation.
The expressions “granules” and “chip” will have the same meaning in the following and will therefore be used as synonyms.
As the manufacture of granules having an I.V. above 0.65 dl/g, in particular in conventional autoclaves, is hardly possible and high viscosities of >0.80 dl/g require special reactors, and moreover the polyesters for food packaging require a very low acetaldehyde content, according to prior art the melt polycondensation was in the past followed by a solid state polycondensation (SSP) leading to an increase in I.V. in general by 0.05-0.4 dl/g and to a reduction of the acetaldehyde content of about 25-100 ppm to values of <1 ppm in the PET (polyethylene terephthalate).
In this solid state polycondensation following the melt polycondensation stage, the medium viscosity is thus increased such that the strengths required for the corresponding field of application are achieved, the acetaldehyde content in food packaging is reduced corresponding to the requirements and the exiting oligomer portion is reduced to a minimum. Here, it is important that moreover the acetaldehyde, which is bound as vinyl ester and also referred to as depot acetaldehyde, is decomposed to such a degree that during processing of the polyester granules into packaging, in particular into polyester bottles according to the stretch blow moulding and injection stretch blow moulding methods, only a minimum amount of acetaldehyde is re-formed in the polyester. Especially if mineral water is filled into polyester bottles, less than 2 ppm of acetaldehyde, preferably even less than 1 ppm of acetaldehyde, should be contained in the bottle wall of polyethylene terephthalate.
Apart from SSP, methods are known for the dealdehydization of polyethylene terephthalate by treatment with nitrogen or dry air, as described in the U.S. Pat. No. 4,230,819. To obtain the required low acetaldehyde content in the material, temperatures of up to about 230° C. are used, while at low temperatures between 170 and 200° C. unsatisfactorily high acetaldehyde contents remain in the granules. When air is used, a strong thermooxidative decomposition of the polyester has to be expected at such high temperatures. When nitrogen is employed, the costs for gas and complicated purification are increased.
In U.S. Pat. No. 4,223,128, temperatures above 220° C. are excluded when air is employed as the carrier gas. The desired increase in I.V. by means of large amounts of dry air with a dew point of −40 to −80° C. is described. At the treatment temperature of 200° C. given in the examples of this patent, in continuous methods comprising a more or less wide range of residence times, oxidative damage of individual grains of the granules cannot be excluded.
In SSP, a chain extension of the polyesters in the solid state for keeping the side reactions, occurring to a stronger degree in a melt, as small as possible, and a removal of the harmful by-products are achieved. With this chain extension expressed in an increase in I.V., products, such as bottles or wire cord requiring an increased strength, can be produced. However, since polyesters are partly crystalline thermoplastic materials, they comprise, depending on their type, a more or less high amorphous proportion. This fact poses difficulties in the execution of SSP as the amorphous proportions at the temperatures necessary for SSP lead to agglomerations which can lead to a standstill of the production plant.
Therefore, it is moreover known to carry out, as a pre-stage to SSP, a crystallization of the partly crystalline chips from the melt polycondensation for preventing tendency to agglomeration under nitrogen or air atmosphere at temperatures between 160-210° C., as described in the U.S. Pat. Nos. 4,064,112, 4,161,578 and 4,370,302.
WO 94/17122 discloses a 2-stage crystallization with preheating and intermediate cooling before SSP to prevent agglomeration. The described SSP temperature is in the range between 205 and 230° C.
To improve the quality of the chips one can, as described in JP 09249744 or U.S. Pat No. 5,663,290, work with moist inert gas before or during SSP, or, as disclosed in U.S. Pat. No. 5,573,820, the chips can be treated beforehand with hot water or directly intensively with water vapour at temperatures up to 200° C. before crystallization. In this case, however, already at the common temperatures of >190° C., one has to expect a strong undesired decrease in I.V. due to the hydrolysis in the PET.
A further method is the treatment of the chips to be crystallized with purified non-dried nitrogen from SSP in countercurrent flow in a second crystallization stage, as illustrated in EP 222 714. The effect for reducing the acetaldehyde content described therein is rather considered as being low.
Processors of these granules are mainly manufacturers of hollow bodies. In preforming machines working according to the injection moulding method preforms are often manufactured from which in turn polyester bottles are produced in a blow moulding method in a further step. Other forming methods for polyester granules, for example in machines for film and foil manufacture, are also possible.
Methods for the manufacture of a polyester with the desired viscosity avoiding SSP have also been developed in the meantime. Thus, a method is for example described in DE 195 03 053 in which the melt exiting from the polycondensation reactor is mixed with an inert gas and an AA-reduced not easily volatilized amide compound in a line provided with static mixing elements, and the melt is fed to a forming device for the manufacture of preforms under vacuum degassing within the shortest-possible time and with as little shearing as possible.
In DE 195 05 680, an inert gas is added to the polycondensation melt having an I.V.=0.5-0.75 dl/g, the melt is polycondensed in an aftercondensation reactor under vacuum until a viscosity of 0.75-0.95 dl/g is reached, and the melt is afterwards immediately supplied to a mould.
EP 0 842 210 states another possibility of avoiding SSP. There, the melt polycondensation is performed until a viscosity of 0.65-0.85 dl/g is reached, the polyester is cooled and granulated, fused again and then freed from volatile substances, such as AA, in a suited device by rinsing with a suited cleansing agent.
If the polyester melt is not directly taken from a forming unit, it is generally granulated to obtain an intermediate product that can be stored and transported.
For the granulation of plastics, for example the pelletization method has been introduced to the market. This method is characterized by pressing relatively long plastic strands through a breaker plate in continuous operation and subsequently passing them in a freely hanging state through a water bath after a short distance of transport through air. Due to the small surface of a plastic strand compared to the granules, water absorption can here be largely restricted. The cooled strands are dried and fed to a granulator. In this method, the granulation is performed in a solid state and cylindrical chips are obtained. Subsequently, normally another drying takes place, for example described in DE 43 14 162 or in the plastics handbook. When this granulation method is used, the possibility of a strong temperature increase at points in the strand and thus of increased decomposition phenomena in the polymer and irregular degrees of crystallization varying from chip to chip are very high. With this technology, cooling in the chip takes place from the outside to the inside.
Another possibility of granulating polymer melt after polycondensation is today above all underwater granulation where the melt is directly separated with cutting knives after it exits from the nozzles/breaker plates of the granulator in a subsequent water chamber. The separated granules are still plastic and are deformed by surface tension when they are quenched in cold water, where the cooling also takes place from the outside to the inside, and they take an approximately round to lenticular contour. The cooled granules are separated from the water flow in a water separator, for example a centrifuge, and dried and then packed into big bags or conveyed into silos for their further processing (DE 35 41 500, DE 199 14 116, EP 0 432 427, DE 37 02 841). The chips manufactured in this manner comprise a uniform degree of crystallization of less than 10%.
In U.S. Pat. No. 4,436,782, a method of granulation and further treatment of PET to chips is in turn described in which at temperatures between 260° C. and 280° C. an oligomer mixture of an I.V. of 0.08-0.15 dl/g is pressed through nozzles, so that drops are formed which fall through a cooling zone with inert gas atmosphere into a water bath or onto a belt conveyor, and the drops solidify to form amorphous chips. In this method, too, chips having a high proportion of amorphous structures are formed.
In all described methods, granules with a low degree of crystallization, usually below 12%, are obtained. It is well-known that expensive reaction steps are necessary to increase the crystallinity of the polymeric granules for example as pre-stage to SSP. High operation costs arise among others due to the fact that the granules arriving at ambient temperature first have to be heated to crystallization temperature.
In WO 01/81450, describe a method and device for dropping pre-products of thermoplastic polyesters and copolyesters which overcome the disadvantage of the above-described granulation methods concerning crystallinity, describe the shortening of the process of conventional granulation methods and are based on presently known procedure steps and devices to manufacture surface-crystallized dropped pre-products in the form of monomers, oligomers, monomer-glycol mixtures or partly polycondensed materials. To this end, the product is introduced into a gaseous medium wherein the gaseous medium accelerates the crystallization process of the pre-product and induces the crystallization state in an accelerated manner after the dropped pre-product has entered the gaseous medium by keeping the dropped pre-product at a temperature of >100° C. and below its melting point for a limited period until sufficient crystallization in the surface of the drop is terminated. That means, here too, the outer layer is crystallized to a greater extent. Thus, a non-sticking surface promising direct further treatment to form high-polymeric condensation polymer is obtained. A material produced in such a way only withstands the required mechanical loads to a certain level. Brittleness is increased compared to an amorphous chip. Another disadvantage of this crystallinity generation in the low-molecular range is that the chip is, after SSP is terminated, perfectly crystallized through with rigidly oriented crystal structures for the destruction of which incomparably higher energy is required during the melting operation when, for example, preforms are manufactured with injection moulding. Due to the required high melting temperature of at least about 300° C., acetaldehyde reformation in the preforms strongly increases and quality is deteriorated, which is mainly also due to increased decomposition reactions. Moreover, there is a risk that the course of SSP is hindered or even stopped due to the immobility of the chain ends when viscosity increases.
A further granulation method for the manufacture of crystallized chips during the granulation process is described in WO 01/05566. Melted plastic strands exiting from nozzles are directly partly crystallized in a temperature-controlled liquid medium on a crystallization line, where in this liquid medium temperatures are kept above the glass transition temperature of the plastic strands. The granulation device is downstream of the crystallization line. By the crystallization in the shell of the plastic, sufficient strength is present to subsequently divide the plastic strands into chips after a short temperature-controlled line in the granulation system without previous drying. That means, here too, the outer layer is crystallized to a greater extent. A disadvantage is that after granulation of the plastics, there is a mixture of granulate and liquid medium, and thus drying of the granules with known means has to be effected.
The German patent application DE 103 49 016 A1 as well as WO 2005/044901 A1 describe that directly after an underwater granulation, the just manufactured chips are very quickly freed from water and dried and crystallized utilizing their sensible heat. In the process, nearly spherical chips are obtained. To avoid agglomeration of the chips, these are conveyed to a downstream filling plant or a further processing plant directly after the water has been removed by centrifugation and after sufficient residence time via a vibroconveyor or oscillating conveyor. With this technology, the crystallization process takes place from the inside to the outside of the chip with utilization of their sensible heat, whereby a more uniform crystallization across the diameter of each chip is achieved. Commercial plants produce about 600 tons of polyester granules per day. To crystallize them according to the method described in DE 103 49 016 A1 or WO 2005/044901 A1, for mechanical reasons several parallel vibroconveyors or oscillating conveyors are necessary, causing considerable investment and operation costs.
The chips manufactured by means of this method can be further treated in a conventional plant for solid state dealdehydization according to DE 102004015515 A1 to reduce the acetaldehyde content to a desired value. The chips still having a heat of 140° C. after crystallization have to be heated to a temperature of more than 210° C. for solid state dealdehydization. In this manner, the acetaldehyde content in the chip can be reduced, for example, from 43 ppm to 0.9 ppm.
DE 10158793 A1 describes a two-stage crystallization method in which in the first stage, a so-called fluid-bed crystallizer, partly crystalline polyester material having a degree of crystallization of about 40 to about 48% is provided, and in a second stage, a so-called shaft-type crystallizer, the partly crystalline polyester material flows at temperatures appropriate for crystallization in a zone (i) under mechanical disturbance and in countercurrent flow, in a following zone (ii) under mechanical disturbance and gas in co-current flow, and in a third zone (iii) without mechanical disturbance and gas in co-current flow. After the second stage, the polyester material has a degree of crystallization of about 49 to about 53%. The preferred total residence time in both stages is 130 to 330 min. While in the shaft-type crystallizer practically no dust is formed due to its design, in the fluid-bed crystallizer a lot of dust is formed due to the intensive movement of the chips against one another, which dust is removed from the fluid bed by the turbulence gas flow and has to be separated in the waste gas purification system. The turbulent gas flow guidance moreover requires complicated and energy-intensive means, such as for example blowers, tubes and heat exchangers.
The aim of these crystallization steps is to reduce the amorphous proportion of the polyester to such an extent that SSP or dealdehydization can be performed without agglomerations.
The basic differences between the execution of SSP or dealdehydization and crystallization are:                1. The residence times in crystallization are considerably shorter than in SSP (on average 2-180 min compared to 5-40 h).        2. In crystallization, the physical processes prevail compared to chemical reactions, for example, in crystallization, the I.V. only slightly increases (by 0.01 to 0.02 dl/g), while in SSP, the increase of I.V. is normally 0.2 to 0.3 dl/g.        
That is, the disadvantages of the known crystallization methods are in particular the cost- and energy-intensive devices as well as the high amount of dust.